1. Pop one’s clogs
You don’t get much more British than this. To pop one’s clogs is a euphemism for dying or death.
Example: “No one knew he was about to pop his clogs.”
2. That went down a treat
If something goes down a treat, then it was thoroughly enjoyed.
Example: “That cake went down a treat.”
3. Take the mickey
Us Brits love to make fun of and tease each other and that’s exactly what ‘taking the mickey’ means. You can also say ‘take the mick.’
Example: “Stop taking the mickey out of your brother.”
4. Itchy feet
This refers to when you want to try or do something new, such as travelling.
Example: “After two years in the job she’s got itchy feet, so she’s going to spend three months in Australia.”
5. At a loose end
If you’re at a loose end, it means you’re bored or you have nothing to do.
Example: “He’s been at a loose end ever since he retired.”
6. Another string to your bow
This means to have another skill that can help you in life, particularly with employment.
Example: “I’m learning French so I’ll have another string to my bow.”
7. As the actress said to the bishop
This is the British equivalent of ‘that’s what she said.’ It highlights a sexual reference whether it was deliberate or not.
Example: “Blimey, that’s a big one — as the actress said to the bishop.”
8. Bob’s your uncle (and fanny’s your aunt)
This phrase means that something will be successful. It is the equivalent of ‘and there you go,’ or as the French say ‘et voilà!’ Adding the ‘and fanny’s your aunt’ makes you that much more British.
Example:
    A: “Where’s the Queen Elizabeth Pub?”
    B: “You go down the road, take the first left and Bob’s your uncle — there it is on the corner!”
9. Cheap as chips
We love a good bargain, and when we find one we can’t help but exclaim that it’s ‘as cheap as chips.’
Example: “Only a fiver for a ticket — cheap as chips mate!”
10. Look after the pennies and the pounds will look after themselves
This is one that our grandparents have told us our whole lives. If you take care not to waste small amounts of money, then it will accumulate into something more substantial.
11. Nosy parker
This is for all the nosy people of the world. A ‘nosy parker’ is someone who is extremely interested in other people’s lives.
Example: “Stop being such a nosy parker! They’re having a private conversation!”

Taken from http://matadornetwork.com/abroad/11-idioms-brits-understand/